1. Field
The aspects of the present disclosure relate generally to traffic signals and in particular to dimming of LED traffic signal lamps.
2. Description of Related Art
In recent years, there has been a great deal of effort directed towards developing lighting systems that use less electrical power. A significant amount of energy is wasted by present commercial traffic control lighting systems when operated at a single level of power. The high light output typically required for visibility during daylight operation greatly exceeds that which is required for visibility during the nighttime hours and can also create excessive amounts of glare for drivers approaching the signal light at night leading to unsafe driving conditions. To overcome these problems, street traffic controllers have been developed that are capable of controlling the light level of signal lights such that full brightness is used during daylight hours, and a dimmed, more energy efficient and less glaring light level is used during nighttime.
Early street traffic controllers were designed for use with incandescent signal lamps that were typically operated directly off the local mains power, such as the 110 volt, 60 Hertz grid power available in North America. A common approach used to dim incandescent signal lamps is to use a form of pulse width modulation known as phase control dimming to adjust the amount of power delivered to the incandescent signal lamp. In phase control dimming a gated diode like device, such as a silicon controlled rectifier (SCR), thyristor, or triode for alternating current (TRIAC), is used to shut off the current each time the mains voltage, which is used to supply power to the lamp, changes direction. The current is then turned back on a variable amount of time (phase angle) after the start of each alternating current half cycle, thereby controlling the amount of power delivered to the lamp. Various forms of phase control dimming are used in traffic controllers that all have the characteristic of turning the lamp power signal off for a portion of each half cycle of the input power. Common forms of phase control dimming are referred to as forward phase control dimming and reverse phase control dimming.
FIG. 1 illustrates graphs 101, 102 of lamp power signals produced by forward phase control dimming. Each graph shows voltage along the vertical axis and time along the horizontal axis. In forward phase control dimming, the lamp power signal is turned on part way through each half cycle of the input power. By controlling the point or phase angle at which the lamp power signal is turned on, the amount of power contained in the lamp power signal may be varied. Graph 101 illustrates a lamp power signal produced by a load switch comprising a TRIAC device configured to provide forward phase control dimming. In certain load switches the input power is rectified along with phase control resulting in a DC forward phase control dimming signal as shown in graph 102. Such a lamp power signal 102 may be produced by a load switch comprising a rectifier and a SCR device. Alternatively, reverse phase control dimming may be used in a traffic signal load switch.
Reverse phase control dimming is where the lamp power signal is turned off for an amount of time before the end of each half cycle as illustrated by graph 201 shown in FIG. 2. Each graph in FIG. 2 shows voltage along the vertical axis and time along the horizontal axis. Reverse phase control dimming may also be applied along with rectification resulting in a reverse phase control dimming signal as illustrated in graph 202. With any of these phase control signals 101, 102, 201, 202, the resulting lamp power signal may be viewed as a pulse width modulated signal where the duty cycle is related to the signal's power content. For the purposes of this disclosure, the duty cycle of a phase clipped AC signal, such as those shown in graphs 101, 102, 201, and 202, is defined as the ratio of on-time, i.e. the amount of time where the voltage is non-zero, to off-time, i.e. the amount of time where the voltage is substantially zero, and phase clipped signals such as those illustrated in FIGS. 1 and 2 are referred to as phase control dimming signals.
Replacing incandescent signal lamps with Light Emitting Diode (LED) light sources has become a commonly used approach for reducing energy consumption. It has been long known that LED lights or LED lamps can provide significant reductions in the amount of electricity consumed by traffic signaling applications and also provide better reliability, lower heat generation, improved vibration resistance, and longer lifetimes of the traffic signaling. LED replacement lamps typically comprise an array of individual LED elements arranged in a circular pattern such that the unit is the same size as a standard incandescent traffic signal lamp. By using arrays with a large number of individual elements, failure of several individual elements can be tolerated before the lamp unit needs to be replaced leading to longer overall lamp lifetime. This gradual failure mode also avoids the unsafe condition resulting from failure of an incandescent lamp, such as when the red stop lamp burns out.
It is expensive, and can take extended periods of time, to replace an entire signaling system, including controller, wiring, and light fixtures, with signaling systems designed for use with LED lamps. An attractive alternative to replacing the entire system is to create LED replacement signal lamps that are both physically and electrically compatible with current incandescent signal lamp standards, allowing the more efficient and reliable LED replacement lamps to be retrofit directly into older systems without making any other changes to the older systems. This also allows for a gradual upgrade of older systems by installing an LED replacement lamp each time an older incandescent lamp burns out. The LED light elements used in these replacement signal lamps require low level DC power, often around 12 volts DC. Small switching power supplies are typically included in LED replacement signal lamp assemblies to convert the AC mains voltage supplied by existing traffic control systems into the low level DC voltage required by LED light elements. Unfortunately, the switching power supplies used in LED replacement lamps regulate the lamp power internally and need only a small amount of input power. Consequently LED lamps that include switching supplies produce the same light level even when the input power is reduced by phase control dimming. LED light elements used in these replacement lamps have the capability of varying their brightness according to the amount of current applied, but due to the nature of the switching supplies used these capabilities are largely unused. Thus, the dimming capabilities of the existing street traffic controllers are nullified by the LED replacement lamps. Therefore, there exists a need for LED replacement signal lamps that provide dimming capabilities similar to incandescent lamps.
Accordingly, it would be desirable to provide a system that addresses at least some of the problems identified above.